The present invention relates to improvements in a negative resistance element utilizing a semiconductor material such as GaAs having a negative electroconductivity in a high electric field.
More particularly, it is directed to enlarge the negative resistance region of such semiconductor material.
It has been found that a semiconductor material such as GaAs having two deep negative portions (valleys) within its electroconductive zone has such a property that, when a high electric field (over about 3 KV/cm for GaAs) is applied to the semiconductor material, the electron velocity is rather decreased. However, it is not always true that an element composed of the abovementioned semiconductor material and ohmic electrodes attached thereto exhibits a voltage-current characteristic which is constantly negative in differentiation. Rather, it has been generally considered that such voltage-current characteristic does not show a negative differential resistivity.
It has been theoretically confirmed that, as the semiconductor elements of hitherto known types had a constant cross-sectional area, no negative resistance characteristic appears. The only fact known heretofore is that, when the product of the doping (impurity concentration) n of the material constituting the semiconductive element and the distance l between electrodes of the element is greater than a certain critical value such as, for GaAs, 10.sup.12 cm.sup.-.sup.2, a dipole domain 4 having a high electric field is produced in the element as shown in FIG. 3 and migrates from the cathode side toward the anode side, whereby a so-called Gunn-effect emerges to cause current oscillation due to gradual diminution and disappearance of the dipole domain thus produced. However, when nl is smaller than a certain critical value, e.g. 10.sup.12 cm.sup.-.sup.2 for GaAs, the dipole domain is not produced, with the result that the electric field distribution within the semiconductor element is strengthened in the anode region as shown in FIG. 4, so that the voltage-current characteristic of the element does not show any negative resistance characteristic.
As described above, since, in the relationship of nl &lt; 10.sup.12 cm.sup.-.sup.2 for GaAs, no dipole domain having a high electric field is produced, it has been attempted to amplify microwaves by the use of such element. In this case, however, as the electric field along the element becomes higher than its critical electric field, and the region showing negative electroconductivity is limited to only the region near the anode, the region capable of causing amplification is limited to such region having the negative electroconductivity. Accordingly, the element having the oscillation effect is not always effective for the purpose of amplification.